1. Field of Invention
The present invention relates to a light detecting circuit. More particularly, the present invention relates to a laser distance measuring circuit, which can reduce the current saturation phenomena by the reflection of inner side of laser.
2. Description of Related Art
Generally, distance from an object can be measured with an electronic distance measuring meter. The electronic distance measuring meter needs a reflecting prism or a reflecting sticker to measure the distance more accurately.
A single tube laser distance measuring meter does not need a reflecting prism or a reflecting sticker to measure the distance. The single tube laser distance measuring meter uses a reflecting laser to measure the distance from an object. After the single tube laser distance measuring meter emits a laser beam, the laser beam is reflected from the object surface. The single tube laser distance measuring meter uses an avalanche photo diode (APD) to detect the reflected laser beam and generate responding current. Then, a current to voltage transformer transforms the responding current from the APD to a responding voltage. Next, messages included in the responding voltage are evaluated to determine the distance of an object.
The paths of the emitted laser beam and received laser beam of the single tube laser distance measuring meter are on the same axis. When the laser beam is emitted, the laser beam passes through the prism group in the single tube laser distance measuring meter first. Because the refraction indexes of each prism in the prism group are not the same, a part of the emitted laser beam would be reflected, which results an error in the measurement. The APD receives the reflected part of the emitted laser beam and produces a current saturation phenomenon, which affects the shortest measuring distance of the single tube laser distance measuring meter.
To solve the aforementioned problem, an improved Trans Impedance Amplifier (TIA) circuit to refine the short distance measuring function of the laser distance measuring meter is used. However, if the TIA circuit is implemented, the accuracy of measuring the long distance is sacrificed.